1. Field of the Invention
The present invention relates to a circuit arrangement for serial data transmission between a plurality of subscriber stations comprising at least two data transmitters which have their output terminals connected to the same data bus having two leads and respectively having a control input connected to a control arrangement and controllable by the control arrangement such that, at most, one of the data transmitters is activated at a given time. As a result, in the activated condition, a data transmitter outputs a voltage of a first polarity or a second polarity to the data bus dependent on the state of a data signal applied to the input and has a higher internal resistance at the output in the passive condition than in the active condition. At least one data receiver is connected to the data bus which, constructed as a differential amplifier, assumes one of two prescribed signal states at its output given an existing input voltage which is dependent on the polarity thereof.
2. Description of the Prior Art
A circuit arrangement of the type generally set forth above is already known from the publication "Elektronik", 6/23 March 1984, page 24 and 2/15 June 1984, pages 97 and 98.
The known circuit arrangement contains interface modules for serial transmission, each of which modules is composed of a data transmitter and a data receiver and is suitable for full-duplex or half-duplex operation. In half-duplex operation, the output of the data transmitter is directly connected to the input of the data receiver in each subscriber station. The subscriber stations which can be, for example, a computer and peripheral devices connected thereto, are connected to a data bus. The data transmitter and the data receiver in each subscriber station can be driven into an active condition or into a passive condition by control signals.
Since the transmitters, in an active condition, represent a low-impedance voltage source at the output whose voltage is dependent on the logic level of the data signal applied to the input, only one data transmitter can be active at a given time.
To this end, the control signal that drives the data transmitter of a subscriber station into its active condition can be transmitted to all other subscribers and can be used therein to make an activation impossible for the time in which the data bus is occupied. It is thereby obvious to transmit control signals via control lines or via a control bus that is provided in addition to the data bus.